Cemented carbide bodies containing a dispersed oxide in the matrix metal and a process of making



United States Patent C CEMENTED CARBIDE BODIES CONTAINING A DlSPERfiED OXIDE IN THE MATRIX METAL AND A PROCESS OF MAKING Guy'B. Alexander and Ralph K. ller, Wilmington, Del.,

and Geoffrey W. Meadows, Kennett Square, Pa., assignors to E. I. du Pont de Nernours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed Oct. 15, 1963, Ser. No. 316,428'

Claims. (Cl. 29182.7)

This invention relates 'to cemented carbide compositions and more specifically relates to hard metal carbides cemented with ductile metal which is dispersion hardened with colloidally subdivided, refractory particles. Even more specifically, the invention is directed to cemented carbide bodies consisting of tungsten carbide, titanium carbide, or tantalum carbide cemented with metals selected from the group of iron, cobalt, and nickel, and alloys of these metals with each other and with other metals having an oxide stable up to 300 C., the metals being dispersion hardened with colloidal refractory oxides.

Cemented carbides, such as those used as cutting tips for machining metal, are conventionally made by sintering an intimate mixture of tungsten or titanium carbide with cobalt or nickel, respectively. They are produced by the steps of milling or grinding the tungsten carbide and cobalt powder together, until they are very intimately mixed, after which the powder is cold pressed to the desired shape, and this porous but coherent body is then sintered in a reducing atmosphere such as hydrogen, until the porous compact shrinks and porosity is reduced. Such metal bonded, cemented carbides are known to lose their hardness at elevated temperature. For example, the hardness at 600 C. is less than half that possessed by them at room temperature. If a cutting tool made from such cemented carbides becomes hot, especially when used to cut alloys which remain hard at high temperature, its cutting tip is rapidly destroyed. In cutting such refractory alloys, the only alternative has therefore been to reduce the cutting speed from several hundred feet per minute to as low as 25 or 50 per minute, in order to keep the cutting tip cool to retain its hardness.

It has been suggested that dispersion strengthened metals can be used as bonding agents for ceramics providing the metals are active, such as chromium. An active metal has been defined as one whose oxide is not easily reduced by hydrogen. An active metal was believed to be required because it was found that an inactive metal would not bond to the hard or ceramic phase.

We have now found that in the compositions of the present invention the metals wet the specific carbide phases .so that an active metal is superfluous and indeed harmful to other desired properties of products made from the compositions. For example, cemented carbide cutting tools which retain their hardness even at 600 C. can be made by employing dispersion hardened metals for cementing the carbides contained in the compositions of this invention.

Refractory carbides that can be employed in making cemented carbide bodies according to this invention are: titanium carbide, tungsten carbide, tantalum carbide and mixtures of these. In addition to these, there can be added from 1 to 50% by weight of other carbides selected from .the group consisting of zirconium carbide, vanadium carbide, niobium carbide, hafnium carbide and molybdenum carbide. These particulate carbide starting materials used in forming cemented carbide objects have a particle size ranging from 250 to 1000 millimicrons prior to hot pressing as hereafter further described.

Metals employed for cementing the carbides are those selected from the group consisting of iron, cobalt, and nickel, and alloys of these metals with each other and with 3,249,407 Patented May 3, 1966 other metals having an oxide stable up to 300 C. It is further required that the oxides of these alloying metals have a free energy of formation at 27 C. of from 30 to 70 kcal. per gram atom of oxygen. However, it is to be understood that the particular metal or metal alloy chosen depends upon the carbides to be bonded. Thus, with tungsten carbide, cobalt is the preferred metal, although nickel also gives good results.

From 0.1% to 50% by volume of the metal phase'can be used for cementing the carbide. Cemented carbide bodies preferably contain from 1% to 35% by volume of the metal phase while most preferred bodies contain the metal phase in an amount ranging from 5% to 35% by volume.

The dispersion hardening agents to be employed in the metal phasecan be described broadly as refractory colloids. Preferably, refractory oxides are used, and thorium oxide specifically is most preferred. Dispersion strengthened metals employed in this invention are more specifically and completely described in G. B. Alexander et al., U.S. Patent 3,087,234.

The process of producing the intimately mixed metal carbide dispersion hardened metal powder for pressing consists of ballrnilling the hard carbide powder with the dispersion hardened metal powder in an inert liquid. Ballmilling should be carried out in a closed container, and preferably with an inert atmosphere over the inert liquid. During recovery of the powder from the liquid, it is preferred to leave some liquid on the powder prior to transferring it to a reduction furnace for final treatment in hydrogen, as hereafter described, in order to minimize oxidation.

The ballmilling is continued until the specific surface area of the carbide is between 6/D and 24/D m. /g., where D is the density of the carbide in grams per cubic centimeter. This surface area corresponds to a carbide particle size ranging from 250 to 1000 millimicrons. The surface area is determined by recovering the carbide from the mixture by dissolving away the metal with acid and separating the colloidal dispersion hardening agent from the heavier carbide by selective sedimentation and elutriation. In some instances it may be necessary to first determine the specific surface area of the dispersion hardening agent that is present in the metal powder prior to ballmilling. Procedures for this determination are adequately described in Bugosh U.S. Patent 3,019,103 and in Alexander et al. U.S. Patent 3,087,234. The size of the filler particles in the metal is not affected by the ballmilling step.

The ballmills and grinding balls are preferably made of tungsten carbide or of the metal to be used for cementing. If the latter, the metal need not be dispersion hardened for use as part of the grinding equipment. v Mills lined with elastomeric material can also be used.

For this purpose it is important that a type of elastomer be employed that is not attacked or softened by the grinding medium. Thus, for example, it is much preferred to use mills that are lined with neoprene and in this case to grind the .powder in an aliphatic oil or a purified kerosene free from aromatic fractions.

The milled composition of the metal carbide and the dispersion hardened metal binder is heated in a hydrogen atmosphere to a temperature not exceeding about 650 to remove traces of oxide contamination which may have occurred during the milling process. The reduced powder composition which at this stage has a highly reactive surface can then be discharged from the hydrogen furnace into an inert atmosphere such as argon, and loaded into suitable molds for hot pressing. All operations are performed out of contact with air to avoid reoxidation.

Alternatively, after the reduction step, the powder can be partially sintered by raising the temperature briefly 23 to l0501100 C. in an inert atmosphere such as argon. After cooling to room temperature, the powder is exposed slowly to a low concentration of air in argon to prevent it from oxidizing extensively with development of heat by providing a thin protective surface coating of oxide. The deactivated powder can then be handled in the atmosphere and charged to suitable molds for hot pressing. Throughout the processing of the powder-"it is important that it be kept in closed containers, away from the moisture, and at all times handled so as to minimize oxidation.

The intimately mixed carbide dispersion hardened metal powders are best fabricated into bodies by hot pressing, which is accomplished by heating the powder to elevated temperatures under high pressure. Furthermore, the time and temperature of heating must be closely controlled for each particular type of composition being pressed. It is necessary to hot press the mixed powder at a temperature which rapidly eliminates porosity within a few minutes under a pressure of about 4000 pounds per square inch.

Hot pressing is ordinarily carried out in graphite molds under a pressure of 1000 to 5000 pounds per square inch. The temperature of the mold is raised rapidly and the material molded at the minimum temperature which produces a body having a density of at least 98% of theoretical density in a period of about five minutes or so, after which the product is at once rapidly cooled. In most instances it is preferred to load the loose powder into the mold, raise the temperature to the desired point and only then apply the pressure.

The non-porous state of the product is determined by measuring the density by means Well known in the art, and comparing the result with the total density calculated from the densities of the components in the composition.

The invention is further described and will be more readily understood and practiced by reference to the following illustrative examples:

Example 1 A solution of nickel nitrate is prepared by dissolving 4320 g. of nickel nitrate hydrate, Ni(NO -6H O in water and diluting this to liters. Seventy-seven grams of a thoria sol (26% ThO stabilized with a trace of nitric acid and containing substantially discrete particles having an average diameter of about 5 to 10 millimicrons is diluted to 5 liters. To a heel containing 5 liters of water at room temperature, the solution of nickel nitrate, the diluted thoria sol, and an ammonium hydroxide-ammonium carbonate solution are added as separate solutions simultaneously, and at uniform rates, while maintaining very vigorous agitation. During the precipitation the pH in the reaction is maintained at 7.5. A precipitate of nickel hydroxide carbonate is thus deposited with the thoria particles. The resulting mixture is filtered and washed to remove the ammonium nitrate. The filter cake is dried in an oven at 300 C.

The product obtained is-pulverized with a hammer mill to pass 325 mesh, placed in an oven and heated to a temperature of 500 C. Hydrogen is slowly passed over the powder at such a rate that sufficient hydrogen is added to the nickel oxide to reduce it in a period of four hours. The flow of hydrogen is maintained at a steady, uniform rate during this reduction procedure for eight hours. Thereafter, the temperature is raised to 700 C. and the flow of dry, pure hydrogen is greatly increased and finally the temperature is raised to 950 C. to complete the reduction. The resulting product is a nickel powder containing 2 volume percent of discrete, colloidal thoria particles entrapped or dispersedin a network of nickel metal. Because of the presence of the thoria, the nickel grains are extremely small, on the order of 1 micron or even smaller.

Sixty grams of the nickel-thoria powder prepared above,

940 g. of tungsten carbide powder, average particle size 3 microns, 1500 cc. of a low viscosity aliphatic hydrocarbon oil, Soltrol 170, a product of the Phillips Petroleum Company, and 25 pounds of tungsten carbide cylinders, A" diameter and A long are charged to a one gallon polychloroprene elastomer-lined ballmill. The mill is rotated at a speed of 65 r.p.m. for four days. The contents of the mill are then washed out with hexane and the tungsten carbide cylinders are separated from the oil suspension by screening. The solids settle from the suspension on standing and the supernatant oil is poured off. Residual oil is removed from the solids by slurrying with hexane, settling and decanting, three or four times, and the solids are finally dried in a vacuum oven provided with a nitrogen purge at C. The powder is then cooled to 25 C. in the oven before it is exposed to air.

The dry powder is heated in a stream consisting of 5 parts hydrogen and one part argon at 600 C. for twenty-four hours and the temperature is then raised to 1050 C. for one hour. The powder is allowed to cool under argon to room temperature and air is then slowly admitted to the furnace. The discharged powder is then screened to pass 200 mesh. It consists of 6% nickel-thoria and 94% tungsten carbide by weight, 01' 10% and 90% respectively by volume. Both the nickel and tungsten carbide particles are of the order of 1 micron or less.

The nitrogen surface area of the residual tungsten carbide and thoria after dissolving away the nickel by boiling a sample of the powder with 6 N hydrochloric acid is 3.5 m. g. Based on the original nitrogen surface area of the thoria component, 45 m. /g., the thoria component of the residue calculated to contribute a surface area of 2.7 m. g. which means that the tungsten carbide surface area is 0.8 mP/g. corresponding to an average particle size of 500 III/1..

Fifty-five grams of the nickel-thoria tungsten carbide powder are charged to a 1" diameter cylindrical carbon cylinder fitted at both ends with closely fitting carbon pistons. The assembly is charged to a double acting induction heated vacuum hot press and the powder confined under 400 p.s.i. is heated to 1400 C. in seven minutes at which time the pressure on the sample is increased to 4000 p.s.i. and the temperature is held at 1400" C. for about two minutes. The sample is then removed from the hot zone and after cooling a cylindrical disc 1" in diameter and thick is obtained. The disc is then cut up with a diamond saw into a /2" square and several 0.070" square cross section and M1" square cross section bars for physical testing. The body is found to have the theoretical density of 14.96, a transverse rupture strength of 240,000 p.s.i., an impact strength of 60 ft. lb./in. and a Rockwell A hardness number of 91.4. v 1

The /2" square is surface finished by electric arc grinding and each corner is provided with a nose radius of A21! The finished cutting insert is used to trim down a cylinder of a nickel-based super alloy having the composition 58% nickel, 19% chromium, 14% cobalt, 3% molybdenum, 2.5% titanium, 2% iron, 1.2% aluminum and trace amounts of manganese, silicon and carbon at 0.050 depth of cut, a feed rate of 0.010 in./rev., a lead angle of 15, and a surface speed of ft./min. Several corners are found to remove at least two to three times as much metal before failure than the best conventional cobaltbonded tungsten carbide based cutting inserts which were found to remove between 1 to 2 cubic inches of metal before failure.

Example 2 Two hundred grams of the nickel-thoria powder prepared in Example 1, 1000 g. of titanium carbide powder, average size 5 microns, 3500 cc. of Soltrol and 80 lb. of nickel balls, A" to /2" in diameter are charged to a four gallon nickel mill. The mill is rotated at a speed of 45 rpm. for five days.

The product, recovered and reduced under hydrogen as described in Example 1, consists of 16.5% nickel-thoria and 83.5% titanium carbide by weight both the nickel and titanium carbide particles being of the order of one micron or less.

The nitrogen surface area of the residual titanium carbide and thoria after dissolving away the nickel by boiling a sample of the powder with 6 N hydrochloric acid is 5.3 mfilg. Based on the nitrogen surface area of the thoria component, 6.5 m. /g., the thoria component of the residue calculated to contribute a surface area of 0.5 m. /g., which means that the titanium carbide surface area is 4.8 m. /g., corresponding to an average particle size of 255 m Twenty grams of the nickel-thoria titanium carbide are hot pressed by the procedure of Example 1 with the difference that the pressing temperature is 1500 C. The resulting body is fabricated to give a cutting insert and bars for physical testing as described in Example 1. The measured density is 5.30, 99.6% of the theoretical value, the transverse rupture strength is 270,000 p.s.i., the impact strength is 115 ft. lb./in. and the Rockwell A hardness number is 92.3.

A /2 square cutting insert prepared and tested as described in Example 1 removes at least twice as much metal before failure than does'a conventional cobaltbonded tungsten carbide cutting insert.

Example 3 A procedure similar to that of Example 1 is employed to prepare a powder comprising volume percent of discrete, colloidal thoria particles dispersed in a network of cobalt metal. The aqueous solutions used in the precipitation step are 3380 g. of cobaltous acetate hydrate, Co(C H O -4H O in 5 liters and 385 grams of the thoria sol of Example 1 is 5 liters. A further modification of the procedure of Example 1 is that the temperature is raised to 1050 C. to complete'the reduction.

A powder consisting of 6% cobalt-thoria and 94% tungsten carbide by weight is prepared by .the milling procedure of Example 1. The cobalt powder particles are on the average 1 micron in size and the tungsten carbide particles are of the order of 1 micron or less.

, Fifty-five grams of the cobalt-thoria tungsten carbide powder are hot pressed by the procedure of Example 1.

The resulting body is fabricated to give a cutting insert and bars for physical testing as described in Example 1. The measured density is 14.95, 99.9% of the theoretical value, the transverse rupture strength is 360,000 p.s.i., the impact strength is 100 ft. lb./in. and the Rockwell A hardness number is 92.6.

A /2" square cutting insert prepared and tested as in Example 1 removes at least two to three times as much metal before failure than does a conventional cobaltbonded tungsten carbide cutting insert.

Example 4 A composition consisting of 6% cobalt containing 10 volume percent dispersed thoria, 89% tungsten carbide and 5% tantalum carbide is prepared by the procedure of Example 3, with the addition of 250 mesh tantalum carbide in the ballmilling step.

The resulting hot pressed body is found to have a density of 14.92, 99.9% of the theoretical value, the transverse rupture strength is 350,000 p.s.i., the impact strength is 130 ft. lb./in. and the Rockwell A hardness number tional cobalt-bonded tungsten carbide-titanium carbide steel grade cutting tool which removed 6 /2 cubic inches same cutting conditions.

Example 5 A composition consisting of 6% nickel containing 2 volume percent dispersed thoria, 84% titanium carbide and 10% molybdenum monocarbide is prepared by the procedure of Example 2, with the addition of 325 mesh molybdenum carbide in the ballmilling step.

The resulting hot pressed body is found to have a density of 5.30, 99.85% of the theoretical value, the transverse rupture strength is 225,000 p.s.i., the impact strength i9s1990 ft. lb./in. and the Rockwell A hardness number is A. /2 square cutting insert prepared and tested as described in Example 1 removes two to three times as much metal before failure than does the standard cobalt-bonded tungsten carbide cutting insert.

We claim:

1. A cemented carbide body having a density of at least 98% of theoretical density consisting essentially of at least one particulate metal carbide selected from the group consisting of titanium carbide, tungsten carbide and tantalum carbide, and a matrix metal selected from the group consisting of iron, cobalt and nickel and alloys of these metals with each other and with other metals having an oxide stable up to 300 C., said oxide having a free energy of formation at 27 C. of from 30 to 70 kcal. per gram atom of oxygen, said matrix metal having uniformly dispersed therein from 0.5% to 10% by volume of a plurality of discrete metal oxide filler particles having an average size of 5 to 1000 millimicrons, said metal oxide .filler particles having a free energy of formation at 1000 C. of above 60 kcal. per gram of oxygen and a melting point above 1000 C.

2. The cemented carbide body of claim 1 wherein said filled matrix metal is present in an amount ranging from 0.1% to by volume.

3. The cemented carbide body of claim 1 wherein said filled matrix metal is present in an amount ranging from 1% to 35% by volume.

4. The cemented carbide body of claim 1 wherein said filled matrix metal is present in an amount ranging from 5% to 35% by volume.

5. The invention of claim 1 wherein said metal carbide is tungsten carbide, said matrix metal is cobalt and said metal oxide filler is thoria.

6. A process for making a cemented carbide body having a density of at least 98% of theoretical density comprising the steps of ballmilling-in an inert liquid a metal carbide selected from the group consisting of titanium carbide, tungsten carbide and tantalum carbide together with a matrix metal selected from the group consisting of iron, cobalt and nickel and alloys of these metals with each other and with other metals having an oxide stable up to 300 C., said oxide having a free energy of formation at 27 C. of from 30 to 70 kcal. per gram atom of oxygen, said matrix metal having uniformly dispersed therein from 0.5% to 10% by volume of a plurality of discrete metal oxide filler particles having an average size of 5 to 1000 millimicrons, said metal oxide filler particles having a free energy of formation at 1000 C. of above kcal. per gram atom of oxygen and a melting point above 1000 C., continuing said ballmilling for a period sufficient to increase the-specific surface area of the metal carbide to a value ranging from 6/D to 24/D square meters, where D is the density of the metal carbide in grams per cubic centimeter, hot pressing said ballmilled metal carbide filled matrix metal powder at a pressure ranging from 1000 to 5000 p.s.i. and at the minimum temperature sufficient to produce a cemented carbide body having a density of at least 98% of theoretical density in a period of time ranging from about 5 to 10 minutes after the application of said pressure, and thereafter rapidly cooling said hot-pressed carbide body.

7. The process of claim 6 wherein said metal carbide is tungsten carbide, said matrix metal is cobalt and said metal oxide filler is thoria.

8. The process of claim 6 wherein said filled matrix metal is introduced into said ballmill in an amount ranging from 0.1% to 50% by volume based upon the total volume of the metal carbide filled matrix metal powder introduced into said ballmill.

9. The process of claim 6 wherein said filled matrix metal is introduced into said ballmill in an amount ranging from 1% to 35% by volume based upon the total volume of the metal carbide filled matrix metal powder introduced into said ballmill.

10. The process of claim 6 wherein said filled matrix References Cited by the Examiner UNITED STATES PATENTS 1,858,300 5/1932 Laise 75202 2,033,513 3/1936 Comstock 75203 2,972,529 2/1961 Alexander et al 75206 LEON D. ROSDOL, Primary Examiner.

REUBEN EPSTEIN, Examiner.

R. L. GOLDBERG, R. L. GRUDZIECKI,

Assistant Examiners. 

1. A CEMENTED CARBIDE BODY HAVING A DENSITY OF AT LEAST 98% OF THEORETICAL DENSITY CONSISTING ESSENTIALLY OF AT LEAST ONE PARTICULATE METAL CARBIDE SELECTED FROM THE GROUP CONSISTING OF TITANIUM CARBIDE, TUNGSTEN CARBIDE AND TANTALUM CARBIDE, AND A MATRIX METAL SELECTED FROM THE GROUP CONSISTING OF IORN, COBALT AND NICKEL AND ALLOYS OF THESE METALS WITH EACH OTHER AND WITH OTHER METALS HAVING AN OXIDE STABLE UP TO 300*C., SAID OXIDE HAVING A FREE ENERGY OF FORMATION AT 27*C. OF FROM 30 TO 70 KCAL. PER GRAM ATOM OF OXYGEN, SAID MATRIX METAL HAVING UNIFORMLY DISPERSED THEREIN FROM 0.5% TO 10% BY VOLUME OF A PLURALITY OF DISCRETE METAL OXIDE FILLER PARTICLES HAVING AN AVERAGE SIZE 2 TO 1000 MILLIMICRONS, SAID METAL OXIDE FILLER PARTICLES HAVING A FREE ENERGY OF FORMATION AT 1000*C. OF ABOVE 60 KCAL. PER GRAM OF OXYGEN AND A MELTING POINT ABOVE 1000*C.
 6. A PROCESS FOR MAKING A CEMENTED CARBIDE BODY HAVING A DENSITY OF AT LEAST 98% OF THEORETICAL DENSITY COMPRISING THE STEPS OF BALLMILLING IN AN INERT LIQUID A METAL CARBIDE SELECTED FROM THE GROUP CONSISTING OF TITANIUM CARBIDE, TUNGSTEN CARBIDE AND TANTALUM CARBIDE TOGETHER WITH A MATRIX METAL SELECTED FROM THE GROUP CONSISTING OF ION, COBALT AND NICKEL AND ALLOYS OF THESE METALS WITH EACH OTHER AND WITH OTHER METALS HAVING AN OXIDE STABLE UP TO 300*C., SAID OXIDE HAVING A FREE ENERGY OF FORMATION AT 27*C. OF FROM 30 TO 70 KCAL. PER GRAM ATOM OF OXYGEN, SAID MATRIX METAL HAVING UNIFORMLY DISPERSED THEREIN FROM 0.5% TO 10% BY VOLUME OF A PLURALITY OF DISCRETE METAL OXIDE FILLER PARTICLES HAVING AN AVERAGE SIZE OF 5 TO 1000 MILLIMICRONS, SAID METAL OXIDE FILLER PARTICLES HAVING A FREE ENERGY OF FORMATION AT 1000*C. OF ABOVE 60 KCAL. PER GRAM ATOM OF OXYGEN AND A MELTING POINT ABOVE 1000*C., CONTINUING SAID BALLMILLING FOR A PERIOD SUFFICIENT TO INCREASE THE SPECIFIC SURFACE AREA OF THE METAL CARBIDE TO A VALUE RANGING FROM 6/D TO 24/D SQUARE METERS, WHERE D IS THE DENSITY OF THE METAL CARBIDE IN GRAMS PER CUBIC CENTIMETER, HOT PRESSING SAID BALLMILLED METAL CARBIDE FILLED MATRIX METAL POWDER AT A PRESSURE RANTING FROM 1000 TO 5000 P.S.I. AND AT THEMINIMUM TEMPERATURE SUFFICIENT TO PRODUCE A CEMENTED CARBIDE BODY HAVING A DENSITY OF AT LEAST 98% OF THEORECTICAL DENSITY IN A PERIOD OF TIME RANGING FROM ABOUT 5 TO 10 MINUTES AFTER THE APPLICATION OF SAID PRESSURE, AND THEREAFTER RAPIDLY COOLING SAID HOT-PRESSED CARBIDE BODY. 